Spreading of oligomers on polymers J.M, Powers1. D.H. Pratten2 S.M. Collard1 G.F. Cowperthwaite3

~Oral Biomaterials UTHSCH, Dental Branch P.O. Box 20068 Houston, TX 77225 2School of Dentistry University of Maryland Baltimore, MD 21203 3Esschem Co. Essington, PA 19029 *Address for correspondence and reprints This study was supported in part by Vivadent (USA), Inc., Tonawanda, NY, and by Esschem Co., Essington, PA Received January 23, 1990 Accepted December 20, 1990 Dent Mater 7:88-91, April, 1991

Abstract-The successful repair of a

composite restoration may depend on the ability of a repair composite to spread on the restoration to be repaired. The purpose of this study was to measurethe spreading of four oligomers on their polymers. The oligomers were: ethoxylated bisphenol A dimethacrylate (EB), BisGMA/3EDMA:70/30(ED), BisGMA-Nupol (NU), and urethane dimethacrylate (UD). Polymer strips were made from these oligomers by lightcuring. Spreading was calculated from measurements of the contact angle of the oligomers on the polymers, the surface tension of the oligomers, and the viscosity of the oligomers. In this model system, values of spreading of EB, ED, UD, and NU on oligomer ED were: 2.8, 1.4, 0.24, and 0.0009 cm/s, respectively. The spreading by oligomers EB, ED, and UD on polymer ED was slightly higher than that on polymer UD.

omposites may require repair as a result of fracture, changes in color, or loss of anatomical form because of chemical and mechanical deterioration. Techniques of mechanical preparation (Boyer et al., 1978) and surface pre-treatment (Puckett et al., 1986) of the old composite for the optimization of repairs with new composite have been reported. Bond strength studies have suggested that no one combination of old resin, new resin, and bonding agent consistently had the highest bond strengths, including repairs involving old and new composite of the same brand (Dhuru and Lloyd, 1986; Pounder et al., 1987). Microfilled resins were shown to be good adhesives to conventional composites of the same resin type and to themselves; however, a urethane dimethacrylate microfilled composite bonded weakly to a Bis-GMA composite (Chan and Boyer, 1983). Measurements of residual unreacted double bonds in composites (Vankerckhoven et al., 1982) have suggested that chemical bonding may be possible during repairs, The wetting of the surface to be repaired by the repair material has been shown to be important to the development of a high repair bond strength (Boyer et al., 1984). The importance of the viscosity of an intermediate resin on bonding has also been studied (Asmussen, 1977; Mount, 1989). The purpose of this study was to use a model system to evaluate the spreading of four oligomers on their polymers by measurements of the contact angle of the oligomers on the polymers, the surface tension of the oligomers by a platinum-blade technique and a critical-surface-tension technique, and the viscosity of the oligomers.

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MATERIALS AND METHODS

The oligomers (Esschem Co., Essington, PA) were ethoxylated bisphenol A dimethacrylate (EB, lot PB1739), BisGMA/3EDMA:70/30 (ED, 88 POWERS et aL/SPREADING OF OLIGOMERS ON POLYMERS

lot 323-34), BisGMA-Nupol 46-4005 (NU, lot 19707-6), and urethane dimethacrylate (UD, lot b1576). Polymer strips 20 mm long, 5 mm wide, and 0.4 mm thick were made from these oligomers by light-curing.

Surface roughness. - T h e roughness (Ra) of the polymer strips was measured with a surface profflometer (Talysurf 10, Taylor-Hobson, Leicester, England LE4 7JQ). The meter cut-off was 0.8 mm, and the vertical magnification was 10,000X. Contact angle. - T h e polymer specimens were cleaned and conditioned according to the following schedule: 10 min in an ultrasonic cleaner in isopropyl alcohol with an amphoteric surfactant (General Purpose Ultrasonic Cleanser, Interstate Dental Equipment & Supply, Inc., Salt Lake City, UT 84115), 10 rain in ultrasonic cleaner with ethanol, one min rinse in distilled water, followed by 24 h in a desiccator. Advancing contact angles (~) of the four oligomers on each of the four polymers were measured in air immediately after removal of the strips from the desiccator at ambient conditions of 23 __ 2°C and 50 __- 10% relative humidity. Contact angles were determined by placement of a single drop of each oligomer on the horizontal surface of each polymer strip. A pipette was used to dispense drops of approximately equal size. The advancing angle of each drop was measured with a telescopic goniometer at times of 30 s and one, two, three, and four rain. Five specimens were evaluated for each of the 16 oligomer-polymer combinations, for a total of 80 measurements at each time interval. Means and standard deviations were calculated. Data were analyzed by analysis of variance with a factorial design (Dalby, 1968). Means were compared with a Tukey interval calculated at the 95% level of confidence (Guenther, 1964). Differences between two means that were larger

than the Tukey interval were statistically significant.

Surface tension-Platinum-blade technique. - T h e surface tension (S) of the four oligomers was measured by a platinum-blade technique and a critical-surface-tension technique. In the first technique, the surface tension was measured at 23°C by use of a platinum blade (#159052, 2.5 cm x 1.0 cm × 0.01 cm, Laboratory Products, Inc., Boston, MA 02136) with a tensiometer (Rosano Surface Tensiometer, Model ST0500MG, Laboratory Products, Inc., Boston, MA 02136). The 15-mL beaker containing each oligomer was deep enough to immerse the platinum blade and wide enough to minimize end-effects. The clean platinum blade was counterbalanced to zero weight on the tensiometer. The blade was immersed vertically in the oligomer and then withdrawn slowly. This cycle took four rain for EB, ED, and UD and 120 rain for NU. The force (F1) to withdraw the blade from the point of total immersion to the point where the lower edge of the blade was in the plane of the liquid surface was measured. The blade was then completely withdrawn from the oligomer, and the weight (F2) of the oligomer adhering to the blade was measured. The difference between these two weights was the corrected force (Fc = F1 - F2). The corrected force was determined five times from five separate immersion and withdrawal cycles. The surface tension was calculated by the equation, S = 0.980 Fc/W, where S is the surface tension in dyne/cm (10 dyne/ cm = Pa cm), Fc is the corrected force in mg, and W is the perimeter of the lower edge of the platinum blade in cm. In this experiment, W was equal to 5 cm.

Surface tension-Critical-surface-tension technique. - T h e surface tension of the oligomers was also determined by a critical-surface-tension technique (Glantz, 1969; Zisman, 1964). Nine organic liquids (benzyl alcohol, diethyl phthalate, ethyl acetate, ethylene glycol, glycerol, m e t h y l propionate, nitrobenzene, 2-propanol, and toluene) were purified by Esschem Co. Four drops of each liquid were placed on a polytetrafiuo-

roethylene slab that was cleaned with methanol. The advancing contact angle was measured at 23°C within one min with a telescopic goniometer on a stage tilted at 15 degrees. The cosine of the advancing contact angle of each organic liquid was plotted vs. its surface tension. A least-squares regression line was obtained from this critical-surface-tension plot. The advancing contact angles of five drops of each oligomer were then measured after four min on the polytetrafluoroethylene slab. The cosines of these angles were used in the calculated regression equation, to give the surface tension of the oligomers.

Viscosity. - T h e viscosity (n) of the four oligomers was measured at 25°C by means of a rotating viscometer (Model 1/4RVT-RL-199, Brookfield E n g i n e e r i n g Laboratories, Inc., Stoughton, MA 02072) at 5 rpm with appropriate t-bar or disk spindles and standard liquids. The viscosity of EB was measured with a 50-mm disk and a 0.393 Pa sec (1 Pa sec = 1000 centipoise) standard (S-200, Cannon Instrument Co., State College, PA 16801). The viscosity of ED was measured with a 48-ram t-bar and the 0.393 Pa sec standard. The viscosity of NU was measured with a 5-ram tbar and 94 Pa sec standard (#091785, Brookfield Engineering Laboratories, Inc., Stoughton, MA 02072). The viscosity of UD was measured with a 25-ram disk and 9.475 Pa sec standard (N3500, Cannon I n s t r u m e n t Co., State College, PA 16801). Five readings for each oligomer and standard were obtained from a pneumatic chart-recorder (Model 5310-E, Foxboro Co., Foxboro, MA 02035). Spreading. -Spreading (R) was calculated as R = S(cos ~)/2n for values of surface tension obtained from both techniques and using the values of the advancing contact angle obtained at four min. RESULTS Mean values of surface roughness (Ra, ~m), with standard deviations in parentheses, were: EB, 0.51 (0.07); ED, 0.31 (0.07); NU, 0.36 (0.07); and UD, 0.30 (0.04), respectively. Mean values and standard deviations of the advancing contact angles

of the oligomers on the polymer strips are reported in Table 1. An analysis of variance showed that significant differences existed among the means. Tukey intervals for comparisons among polymers, oligomers, and times calculated from the analysis of variance were 2.4, 2.4, and 2.8 degrees, respectively, at the 95% level of confidence. All curves of contact angle vs. time decreased with time. The oligomerpolymer combinations demonstrating the smallest decrease (17%) in the contact angle between 30 sec and four rain were ED/UD and EB/UD. The largest decreases (44%) were observed with oligomer-polymer combinations ED/ED and UD/NU. The oligomer-polymer combinations with the lowest contact angles (degrees) for each of the polymers at four rain were: ED/ED, 13.8; EB/NU, 19.7; EB/EB, 20.2; and ED/UD, 32.7. The oligomer-polymer combinations with the highest contact angles (degrees) for each of the polymers at four min were: NU/UD, 72.5; NU/ EB, 72.9; NU/ED, 77.9; and NU/NU, 79.0. Oligomers EB and ED produced the lowest contact angles on the polymers studied. Polymer UD was least easily wetted by all the oligomers except NU. Mean values and standard deviations of the surface tension of the oligomers as measured by the platinum-blade and critical-surface-tension techniques are reported in Table 2. The critical-surface tension of the polytetrafluoroethylene was calculated to be 21.5 dyne/cm. Values of surface tension for the oligomers determined by the critical-surface-tension technique were about 13% higher for EB, ED, and UD but 18% lower for NU than values determined by the platinum-blade technique. Mean values and standard deviations of the viscosity of the oligomers are also reported in Table 2. The ratio N U : U D : E D : E B was 715:12.0:2.1:1.0. The spreading calculated for the platinum-blade and critical-surfacetension techniques for the four oligomers on each of the four polymers is reported in Table 3. Spreading determined from the critical-surfacetension technique was 8 to 20% higher for oligomers EB, ED, and UD on the four polymers but about 19%

Dental Materials/April 1991

89

TABLE 1

CONTACTANGLE vs. TIME FOR FOUROLIGOMERSON FOURPOLYMERS Time Polymer Oligomer 30 s One rain Two rain Three min Four min EB EB 28.3(4.1)* 25.8(4.8) 23.2(2.9) 21.2(3.5) 20.2(4.2) ED 34.7(2.8) 31.2(3.8) 27.9(4.4) 25.6(3.6) 24.7(4.4) NU 113.5(5.6) 102.0(3.6) 85.4(3.3) 77.3(3.2) 72.9(1.9) UD 42.9(3.5) 37.1 (3.8) 31.7(3.8) 28.6(3.7) 27.4(4.2) Eg EB 33.4(7.4) 31.1 (7.6) 27.6(6.2) 26.3(6.6) 25.6(7.0) ED 24.9(2.1) 20.4(1.5) 17.1 (0.7) 14.9(1.1) 13.8(1.1) NU 114.6(14) 102.6(7.6) 93.7(5.0) 83.1 (1.6) 77.9(1.3) UB 46.3(7.9) 39.5(3.9) 33.3(4.5) 30.2(5.5) 27.5(5.7) NU EB 28.7(7.0) 26.0(6.8) 23.2(6.5) 20.9(6.3) 19.7(6.4) ED 28.3 (4.2) 25.6(5.2) 23.2(5.9) 21.7(6.8) 20.5(6.4) NU 112.0(14) 101.1 (11) 87.5(11) 82.1 (12) 79.0(14) UD 47.0(1.7) 40.3(2.6) 32.5(2.6) 28.5(2.5) 26.5(2.8) UD EB 40.7(7.2) 37.0(9.4) 34.5(11) 34.1 (11) 33.6(11) ED 38.8(5.3) 36.0(6.0) 34.2(7.3) 32.8(7.5) 32.2(7.2) NU 105.6(7.9) 100.1 (10) 86.9(9.3) 78.0(8.8) 72.5(8.6) UD 54.0(6.5) 47.0(4.4) 40.3(5.9) 37.6(5.4) 35.8(4.2) *Mean of five replications with standard deviation in parentheses. Tukey intervals for comparisons among polymers, oligomers, and times at the 95% level of confidence were 2.4, 2.4, and 2.8 degrees, respectively. TABLE 2

SURFACETENSIONFOR THE PLATINUM-BLADEAND CRITICAL-SURFACE-TENSIONTECHNIQUESAND VISCOSI~t'OF FOUROLIGOMERS Oligomers EB ED NU UD Property 38.3 (0.3) 54.1 (0.4) 42.5 (0.1) Surface tension, dyne/cm 39.0 (0.1)* (platinum-blade technique) 43.4 (1.4) 44.6 (2.8) 47.9 (1.1) Surface tension, dyne/cm 44.4 (1.4)* (critical-surface-tension technique) 1.54 (0.00) 519 (15) 8.70 (0.14) Viscosity, Pa sec 0.726 (0.005)* *Mean with standard deviation in parentheses of five replications.

TABLE 3

lower for oligomer NU on the four polymers than rates determined from the platinum-blade technique.

SPREADINGCALCULATEDFOR THE PLATINUM-BLADEAND CRITICAL-SURFACE-TENSION TECHNIQUESFOR THE FOUROLIGOMERSON EACHOF THE FOURPOLYMERS Spreading, cm/sec Pt-Blade CST Polymer Oligomer Technique Technique EB EB 2.5 2.9 ED 1.1 1.3 NU 0.0015 0.0012 UD 0.22 0.24 ED EB 2.4 2.8 ED 1.2 1.4 NU 0.0011 0.0009 UD 0.22 0.24 NU EB 2.5 2.9 ED 1.2 1.3 NU 0.0010 0.0008 UD 0.22 0.25 UD EB 2.2 2.5 ED 1.0 1.2 NU 0.0016 0.0013 UD 0.20 0.22

Within four min, all of the oligomers appeared to be approaching equilibrium values of contact angle. Decreases in the contact angle between three and four min ranged from 1.8 to 8.9% for the oligomers studied. Addition (30%) of the diluent triethylene glycol d i m e t h a c r y l a t e (3EDMA) to BisGMA (NU) reduced the viscosity of NU by 99.7%, reduced the contact angle of NU on the four polymers by 56 to 82%, but only slightly reduced the surface tension of NU by 3%. The ratio of the spreading for ED/NU was 1080. The clinical interpretation of this research involving a model system is clearly speculative, but the dominating effect of viscosity on the rate of

9(] POWERS et aL/SPREADING OF OLIGOMERS ON P O L Y M E R S

DISCUSSION

wetting could have clinical implications. The use of a low-viscosity resin as a pre-treatment for a repair may be justified to maximize spreading. Future areas of research include measurement of the critical-surface tension of the polymerized oligomers and further evaluation of the effects of diluents on the surface tension and viscosity of the oligomers. Other factors which could influence spreading in a model system include the amount and composition of filler particles and the effect of the coupling agent. Spreading, as reported here, includes wetting, penetration, solubilization, and swelling of the polymers by the oligomers, each of which may also influence bonding. Bond strength studies of these model oligomers and polymers have not yet been attempted.

REFERENCES ASMUSSEN, E. (1977): Penetration of Restorative Resins into Acid Etched Enamel. I. Viscosity, Surface Tension

and Contact Angle of Restorative Resin Monomers, Acta Odontol Scand 35: 175-182. BOYER, D.B.; CHAN, K.C.; and REZNHARDT,J.W. (1984): Build-up and Repair of Light-cured Composites: Bond Strength, J Dent Res 63: 12411244. BOYER, D.B.; CHAN, K.C.; and TORNEY, D.L. (1978): The Strength of Multilayer and Repaired Composite Resin, J Prosthet Dent 39: 63-67. CHAN, K.C. and BOYER, D.B. (1983): Repair of Conventional and Microfilled Composite Resins, J Prosthet Dent 50: 345-350. DALBY, J. (Programmer) (1968): BMDSV-Analysis of Variance. Ann Arbor, MI: Statistical Research Laboratory, University of Michigan, pp. 1--8. DHURU, V.B. and LLOYD, C.H. (1986): The Fracture Toughness of Repaired Composites, J Oral Rehabil 13: 413421. GLANTZ,P.-O. (1969): On Wettability and Adhesiveness, Odont Revy 20: 1-132. GUENTHER, W.C. (1964): Analysis of

Variance. Englewood Cliffs, N J:

Prentice-Hall, pp. 1-199. MOUNT, G.J. (1989): The Wettability of Bonding Resins Used in the Composite Resin/Glass Ionomer 'Sandwich Technique', Aust Dent J 34: 32-35. POUNDER, B.; GREGORY, W.A.; and POWERS, J.M. (1987): Bond Strengths of Repaired Composite Resins, Oper Dent 12: 127-131. PUCKETT, A.D., JR.; OHARA, J.W., JR.; and HOLDER,R. (1986): The Repair Potential of a Posterior Composite Material, J MS Dent Assoc 42: 12-13, 24. VANKERCKHOVEN, H.; LAMBRECHTS,P.; VAN BEYLEN, M.; DAVIDSON, C.L.; and VANHERLE,G. (1982): Unreacted Methacrylate Groups on the Surfaces of Composite Resins, J Dent Res 61: 791-796. ZISMAN, W.A. (1964): Relation of the Equilibrium Contact Angle to Liquid and Solid Constitution. In: Contact Angle, Wettability and Adhesion. Advances in Chemistry Series No. 43. R. Gold, Ed. Washington, DC: American Chemical Society, pp. 1-51.

Dental Materials~April 1991 91

Spreading of oligomers on polymers.

The successful repair of a composite restoration may depend on the ability of a repair composite to spread on the restoration to be repaired. The purp...
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